Recent deregulation and subsequent open access to the natural gas pipeline industry has strongly encouraged or, in some cases, forced gas businesses toward greater reliance on local energy flow rate measurement. What was once a large, stable, and well-defined source of natural gas is now a composite of many small suppliers with greatly varying gas compositions or involved with gas blending operations. While natural gas still has many advantages and its usage is increasing, it is no longer the inexpensive source of energy that it once was. Under-billing at tariff transfer points can cause revenue losses while over-billing can require accounts receivable corrections that can result in sizable additional costs.
One currently available approach to energy flow measurement uses a gas chromatograph (GC) for composition assay in conjunction with a flow meter. Such measurements are generally cost effective only for large capacity supplies (typically in applications where the volume is on the order of 1 to 30 million scf/day). The capital and the high maintenance costs of GC-based analysis systems can prevent their use in many applications. GC analysis can also be limited to measurements of only clean dry natural gas free of liquids or other contaminants that can foul the GC column. Such conditions are generally not present in a natural gas pipeline. GC systems can also suffer from slow analysis and calculation rates, with a typical analysis cycle requiring four or more minutes of sampling and analysis time. Natural gas moving at a pipeline velocity of 25 ft/sec travels over a mile in four minutes. Thus, a calculation of energy content flow based on GC measurements will in some applications be representative of gas that is already a mile or farther down the pipeline. Operation and maintenance costs of operating GCs can also be quite large due to the required consumable carrier gases and the regular maintenance required to assure that the instrument will continue to provide accurate data.
Another alternative method for measuring the potential energy of natural gas in a pipeline is calorimetry. The flame calorimeter is used to measure the properties of gas reactions. The gases concerned are fed at a known, constant rate to a jet at which the reaction occurs. The reaction chamber and gas pipes are contained in a thermostatically controlled water bath to ensure constant temperature. The reaction is then started and the temperature rise measured after a known amount of gas has been fed into the reaction. Calibration of the calorimeter, either with a standard reaction or by electrical means, allows calculation of the enthalpy (ΔH) of the reaction because the reaction is conducted at constant pressure. Flame calorimetry can be performed in near real time depending on the design of the device employed. However, if the heated mass has a large heat capacity, it will take longer to register a meaningful temperature shift which results in a delay. In addition, calorimeter design is very difficult, especially for processes involving very small energy changes, e.g., energy changes on top of a large background such as pipeline gas. Maintenance and calibration of these devices may also require considerable resources.